A raft of small changes associated with -XConstrainedClassMethods

See Trac #7854.  Specifically:

* Major clean up and simplification of check_op in checkValidClass;
  specifically
     - use checkValidType on the entire method-selector type to detect
       ambiguity
     - put a specific test for -XConstrainedClassMethods

* Make -XConstrainedClassMethods be implied by -XMultiParamTypeClasses
  (a bit ad-hoc but see #7854), and document in the user manual.

* Do the checkAmbiguity test just once in TcValidity.checkValidType,
  rather than repeatedly at every level. See Note [When to call checkAmbiguity]

* Add -XAllowAmbiguousTypes in GHC.IP, since 'ip' really is ambiguous.
  (It's a rather magic function.)

* Improve location info for check_op in checkValidClass

* Update quite a few tests, which had genuinely-ambiguous class
  method signatures.  Some I fixed by making them unambiguous; some
  by adding -XAllowAmbiguousTypes

1 files changed, 6 insertions, 5 deletions

diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index edfdc849bf..8a1c9ec468 100644
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -4442,15 +4442,16 @@ class type variable, thus:
     elem     :: Eq a => a -> s a -> Bool
 </programlisting>
 The type of <literal>elem</literal> is illegal in Haskell 98, because it
-contains the constraint <literal>Eq a</literal>, constrains only the
+contains the constraint <literal>Eq a</literal>, which constrains only the
 class type variable (in this case <literal>a</literal>).
-GHC lifts this restriction (flag <option>-XConstrainedClassMethods</option>).
 </para>
-
-
+<para>
+GHC lifts this restriction with language extension <option>-XConstrainedClassMethods</option>.
+The restriction is a pretty stupid one in the first place, 
+so <option>-XConstrainedClassMethods</option> is implied by <option>-XMultiParamTypeClasses</option>.
+</para>
 </sect3>
 
-
 <sect3 id="class-default-signatures">
 <title>Default method signatures</title>